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19-3181; Rev 0; 2/04
3.2Gbps SFP Transimpedance Amplifiers with RSSI
General Description
The MAX3724/MAX3725 transimpedance amplifiers provide a compact, low-power solution for communication up to 3.2Gbps. They feature 325nA input-referred noise at 2.1GHz bandwidth (BW) with 0.6pF input capacitance. The parts also have >2mAP-P AC input overload. Both parts operate from a single +3.3V supply and consume 93mW. The MAX3724/MAX3725 are in a compact 30-mil x 50-mil die and require no external compensation capacitor. A space-saving filter connection is provided for positive bias to the photodiode through an on-chip 580 resistor to V CC. These features allow easy assembly into a low-cost TO-46 or TO-56 header with a photodiode. The MAX3724 and MAX3748A receiver chip set provides an RSSI output using a Maxim-proprietary* interface technique. The MAX3724 preamplifier, MAX3748A postamplifier, and the DS1858/DS1859 SFP controller meet all the SFF-8472 digital diagnostic requirements.
Features
Wider Bandwidth than MAX3744/MAX3745 RSSI Implementation in 4-Pin TO-46 Header (MAX3724 and MAX3748A) 8psP-P Deterministic Jitter for <100AP-P Input Current 325nARMS Input-Referred Noise at 2.1GHz Bandwidth 28mA Supply Current at +3.3V 2.5GHz Small-Signal Bandwidth 2.0mAP-P AC Overload Die Size: 30 mils x 50 mils (Identical to the MAX3744/MAX3745)
MAX3724/MAX3725
Applications
Up to 3.2Gbps SFF/SFP Optical Receivers Optimized for Small-Form-Factor Pluggable (SFP) Optical Receivers
*Patent pending.
PART MAX3724E/D MAX3725E/D
Ordering Information
TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE Dice** Dice**
**Dice are designed to operate with junction temperatures of -40C to +110C but are tested and guaranteed only at TA = +25C.
Typical Application Circuit
SFP OPTICAL RECEIVER
VCC
400pF 400pF VCC VCC FILTER OUT+ MAX3724 OUTGND IN+ MAX3748A INOUTRSSI 4-PIN TO CAN 3.3k MOD-DEF1
DS1858/ DS1859
0.1F OUT+ 0.1F
HOST BOARD
IN
VCC = 3.3V 4.7k TO 10k
DISABLE
LOS
MOD-DEF2
________________________________________________________________ Maxim Integrated Products
1
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
ABSOLUTE MAXIMUM RATINGS
Power-Supply Voltage (VCC) .................................-0.5V to +6.0V Continuous CML Output Current (OUT+, OUT-) ............................................. -25mA to +25mA Continuous Input Current (IN)...............................-4mA to +4mA Continuous Input Current (FILTER).......................-8mA to +8mA Operating Junction Temperature Range (TJ) ....-55C to +150C Storage Ambient Temperature Range (TSTG) ...-55C to +150C Die Attach Temperature...................................................+400C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V and TA = -40C to +85C. Typical values are at VCC = +3.3V, source capacitance (CIN) = 0.85pF, and TA = +25C, unless otherwise noted.) (Notes 1, 2)
PARAMETER Supply Current Input Bias Voltage Input Overload (Note 3) CIN = 0.85pF, BW = 933MHz CIN = 0.85pF, BW = 2.1GHz (Note 3) Input-Referred Noise (Note 4) IN CIN = 0.85pF, BW = 18GHz (Note 3) CIN = 0.6pF, BW = 933MHz CIN = 0.6pF, BW = 2.1GHz (Note 3) CIN = 0.6pF, BW = 18GHz (Note 3) Differential Transimpedance Small-Signal Bandwidth (Note 3) Low-Frequency Cutoff Deterministic Jitter (Notes 3, 5) Filter Resistance Differential Output Resistance (OUT+, OUT-) Maximum Differential Output Voltage VOD Input > 50AAVE, output termination 50 to VCC (output in limited state) BW Differential output, IIN = 40AAVE -3dB, CIN = 0.6pF -3dB, CIN = 0.85pF -3dB, input current = 20AAVE (Note 3) 100AP-P < input 2.1Gbps, K28.5 pattern 2mAP-P 2.7Gbps, 231 - 1 pattern 10AP-P < input 100AP-P 2.1Gbps, K28.5 pattern 2.7Gbps, 231 - 1 pattern 510 85 220 11 15 8 12 580 100 280 690 115 400 mVP-P 17 2.8 2 1.8 3.5 2.5 2.3 30 33 psP-P 200 325 440 635 4.5 k GHz kHz 2 200 360 485 700 nARMS SYMBOL ICC CONDITIONS Including CML output current (IIN = 0) MIN TYP 28 MAX 41 1.0 UNITS mA V mAP-P
DJ
2
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3.2Gbps SFP Transimpedance Amplifiers with RSSI
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V and TA = -40C to +85C. Typical values are at VCC = +3.3V, source capacitance (CIN) = 0.85pF, and TA = +25C, unless otherwise noted.) (Notes 1, 2)
PARAMETER Single-Ended Output CommonMode Minimum Level (MAX3724) Output Data Transition Time Differential Output Return Loss Power-Supply Noise Rejection RSSI Gain (MAX3724) RSSI Gain Stability (MAX3724) PSNR ARSSI SYMBOL CONDITIONS Relative to VCC, IIN = 1mAAVE Input > 200AP-P 20% to 80% rise/fall time (Note 3) Frequency 1GHz 1GHz < frequency 2GHz IIN = 0 (Note 6) (Note 7) 10log(ARSSI/ARSSI-NOM) where ARSSI-NOM = ARSSI at 3.3V, +25C (Note 3) f < 1MHz 1MHz f < 10MHz MIN 540 TYP 490 70 17 10 46 34 21 0.24 100 MAX UNITS mV ps dB dB A/A dB
MAX3724/MAX3725
Note 1: Dice are designed to operate with junction temperatures of -40C to +110C but are tested and guaranteed only at TA = 25C. Note 2: Source capacitance represents the total capacitance at the IN pad during characterization of the noise and bandwidth parameters. Note 3: Guaranteed by design and characterization. Note 4: Input-referred noise is:
RMS output noise Gain at f = 100MHz
Note 5: Deterministic jitter is the sum of pulse-width distortion (PWD) and pattern-dependent jitter (PDJ). Note 6: Power-supply noise rejection PSNR = -20log(VOUT / VCC), where VOUT is the differential output voltage and VCC is the noise on VCC. Note 7: IOUT _ CM (IIN = 400A) - IOUT _ CM (IIN = 0A) ARSSI = 400A IOUT + + IOUT - where IOUT _ CM = 2 RSSI range is from IIN = 6A to 500A
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
Typical Operating Characteristics
(VCC = +3.3V, CIN = 0.85pF, TA = +25C, unless otherwise noted.)
INPUT-REFERRED NOISE vs. TEMPERATURE
MAX3724/25 toc01
INPUT-REFERRED NOISE vs. TEMPERATURE
BW = 2.1GHz INPUT-REFERRED NOISE (nARMS) 700 600 GAIN (dB) 500 400 300 200
MAX3724/25 toc02
OPTICAL FREQUENCY RESPONSE (70m InGaAS PHOTODIODE, INPUT -22dBm)
35 30 25 20 15 10 5 0
MAX3724/25 toc03
800 UNFILTER INPUT-REFERRED NOISE (nARMS) 700 600 500 400 300 200 -40 -20 0 20 40 60 80 CIN = 0.5pF CIN = 1.5pF CIN = 0.85pF
800
40
CIN = 1.5pF CIN = 0.85pF CIN = 0.5pF
-5 100 -10 100M 1G FREQUENCY (Hz) 10G
100
-40
-20
0
20
40
60
80
TEMPERATURE (C)
TEMPERATURE (C)
DETERMINISTIC JITTER vs. INPUT VOLTAGE
MAX3724/25 toc04
SMALL-SIGNAL TRANSIMPEDANCE vs. TEMPERATURE
MAX3724/25 toc05
EYE DIAGRAM INPUT = 20AP-P, DATA RATE = 2.1Gbps
MAX3724/25 toc06
50 45 DETERMINISTIC JITTER (psP-P) 40 35 30 25 20 15 10 5 0 0.01 0.1 1 2.1Gbps, K28.5 2.7Gbps, 27-1PBRS
75
K28.5 PATTERN
TRANSIMPEDANCE (dB )
70 10mV/div 65
10
60 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
60ps/div
INPUT CURRENT (mAP-P)
EYE DIAGRAM INPUT = 20AP-P, DATA RATE = 3.2Gbps
MAX3724/25 toc07
EYE DIAGRAM INPUT = 2mAP-P, DATA RATE = 2.1Gbps
MAX3724/25 toc08
EYE DIAGRAM INPUT = 2mAP-P, DATA RATE = 3.2Gbps
MAX3724/25 toc09
K28.5 PATTERN
K28.5 PATTERN
K28.5 PATTERN
10mV/div
50mV/div
50mV/div
50ps/div
60ps/div
50ps/div
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
Typical Operating Characteristics (continued)
(VCC = +3.3V, CIN = 0.85pF, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
MAX3724/25 toc11 MAX3724/25 toc10
DIFFERENTIAL S22 vs. FREQUENCY
0 -5 MAX3724 -10 S22 (dB) -15 -20 -25 -30 0 500 1000 1500 2000 2500 3000 3500 4000 FREQUENCY (MHz) MAX3725 70 60 SUPPLY CURRENT (mA) 50 40 30 20 10 0 -40 -20
DC TRANSFER FUNCTION (VFILT = 0V)
150 OUTPUT VOLTAGE (mVP-P) 100 50 0 -50
MAX3724/25 toc12
200
-100 -150 -200
0
20
40
60
80
100
-100
-50
0
50
100
TEMPERATURE (C)
INPUT CURRENT (mAP-P)
RSSI MAX3724, MAX3748A
MAX3724/25 toc13
BANDWIDTH vs. TEMPERATURE
MAX3724/25 toc14
EYE DIAGRAM TEMPERATURE = +100C INPUT = 20AP-P DATA RATE = 2.7Gbps
MAX3724/25 toc15
550 500 MAX3748 RSSI OUTPUT (A) 450 400 350 300 250 200 150 0
TA = -40C
4
223 - 1 PRBS
TA = +85C 3dB BANDWIDTH
3 MAX3724 CIN = 0.6pF 2 MAX3744 CIN = 0.6pF
6mV/div
1
0 500 1000 1500 2000 -40 -20 0 20 40 60 80 100 60ps/div AVERAGE INPUT CURRENT (A) TEMPERATURE (C)
OPTICAL EYE DIAGRAM INPUT = 0dBm, DATA RATE = 3.2Gbps
MAX3724/25 toc16
OPTICAL EYE DIAGRAM INPUT = 0dBm, DATA RATE = 4.25Gbps
MAX3724/25 toc17
OPTICAL EYE DIAGRAM INPUT = -18dBm, DATA RATE = 4.25Gbps
MAX3724/25 toc18
K28.5 PATTERN
K28.5 PATTERN
K28.5 PATTERN
100mV/div
30mV/div
3mV/div
60ps/div
60ps/div
50ps/div
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
Pin Description
MAX3724/ MAX3725 BOND PAD 1, 3 2, 7 4 5 6, 10 8 NAME VCC N.C. IN FILTER GND OUTSupply Voltage No Connection TIA Input. Signal current from photodiode flows into this pin. Provides bias voltage for the photodiode through a 580 resistor to VCC. When grounded, this pin disables the DC cancellation amplifier to allow a DC path from IN to OUT+ and OUT- for testing. Supply Ground Inverting Data Output. Current flowing into IN causes the voltage at OUT- to decrease. For the MAX3724, the common mode between OUT+ and OUT- is proportional to the average input current. Noninverting Data Output. Current flowing into IN causes the voltage at OUT+ to increase. For the MAX3724, the common mode between OUT+ and OUT- is proportional to the average input current. FUNCTION
9
OUT+
MAX3724
RF TRANSIMPEDANCE AMPLIFIER IN 50 DC CANCELLATION CIRCUIT FILTER 50 OUT+ OUTVCC IN RF TRANSIMPEDANCE AMPLIFIER 50
VCC
50 OUT+ OUT-
VCC DC CANCELLATION CIRCUIT FILTER
RSSI
MAX3725
Figure 1. Functional Diagram
Detailed Description
The MAX3724/MAX3725 are transimpedance amplifiers designed for up to 3.2Gbps SFF/SFP transceiver modules. A functional diagram of the MAX3724/MAX3725 is shown in Figure 1. The MAX3724/MAX3725 comprise a transimpedance amplifier stage, a voltage amplifier stage, an output buffer, and a direct-current (DC) feedback cancellation circuit. The MAX3724 also includes a signal strength indicator (RSSI). To provide this signal in
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a standard 4-pin TO header, the RSSI level is added to the common mode of the differential data output pins.
Transimpedance Amplifier Stage
The signal current at the input flows into the summing node of a high-gain amplifier. Shunt feedback through the resistor RF converts this current to a voltage. In parallel with the feedback resistor are two back-to-back Schottky diodes that clamp the output signal for large input currents, as shown in Figure 2.
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
AMPLITUDE AMPLITUDE INPUT FROM PHOTODIODE
TIME OUTPUT (SMALL SIGNALS) INPUT AFTER DC CANCELLATION OUTPUT (LARGE SIGNALS)
TIME
Figure 2. MAX3724/MAX3725 Limited Output
Figure 3. DC Cancellation Effect on Input
VCC 100 OUT+ OUT-
Voltage Amplifier Stage
The voltage amplifier stage provides gain and converts the single-ended input to differential outputs.
DC Cancellation Circuit
The DC cancellation circuit uses low-frequency feedback to remove the DC component of the input signal (Figure 3). This feature centers the input signal within the transimpedance amplifier's linear range, thereby reducing pulse-width distortion caused by large input signals. The DC cancellation circuit is internally compensated and therefore does not require external capacitors.
Output Buffer
The output buffer provides a reverse-terminated voltage output. The buffer is designed to drive a 100 differential load between OUT+ and OUT-. The MAX3724 must be DC-coupled to the MAX3748A. See Figures 4 and 5. For optimum supply-noise rejection, the MAX3725 should be terminated with a matched load. If a singleended output is required, the unused output should be terminated to a 50 resistor to VCC. The MAX3725 does not drive a DC-coupled, 50 grounded load; however, it does drive a compatible 50 CML input.
Figure 4. Equivalent Output MAX3724
VCC
50
50 OUT+ OUT-
Signal-Strength Indicator
The MAX3724 produces a signal proportional to the average photodiode current. This is added to the common mode of the data outputs OUT+ and OUT-. This signal is intended for use with the MAX3748A to provide a ground-referenced RSSI voltage.
Applications Information
Signal-Strength Indicator
The SFF-8472 digital diagnostic specification requires monitoring of input receive power. The MAX3748A and MAX3724 receiver chipset allows for the monitoring of the average receive power by measuring the average DC current of the photodiode.
Figure 5. Equivalent Output MAX3725 7
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3.2Gbps SFP Transimpedance Amplifiers with RSSI
The MAX3724 preamp measures the average photodiode current and provides the information to the output common mode. The MAX3748A RSSI detect block senses the common-mode DC level of input signals IN+ and IN- and provides a ground-level-referenced output signal of the photodiode current. The advantage of this implementation is that it allows the TIA to be packaged in a low-cost conventional 4-pin TO-46 header. The MAX3748A RSSI output is connected to an analog input channel of the DS1858/DS1859 SFP controller to convert the analog information into a 16-bit word. The DS1858/DS1859 provide the received power information to the host board of the optical receiver through a 2-wire interface. The DS1859 allows for internal calibration of the receive power monitor. The MAX3724 and the MAX3748A have been optimized to achieve RSSI stability of better than 2.5dB within the 6A to 500A range of average input photodiode current. To achieve the best accuracy, Maxim recommends receive power calibration to be 6A at the low end, and 500A at the high end of the required range.
MAX3724/MAX3725
Optical Sensitivity Calculation
The input-referred RMS noise current (I N ) of the MAX3724/MAX3725 generally determines the receiver sensitivity. To obtain a system bit-error rate (BER) of 1E-12, the signal-to-noise ratio must always exceed 14.1. The input sensitivity, expressed in average power, can be estimated as: 14.1 x IN (re + 1) Sensitivity = 10log 1000 dBm 2(re - 1) where is the photodiode responsivity in A/W and IN is RMS current in amps.
Input Optical Overload
The overload is the largest input that the MAX3724/ MAX3725 can accept while meeting deterministic jitter specifications. The optical overload can be estimated in terms of average power with the following equation: 2mARMS (re + 1) Overload = 10log 1000 dBm 2(re - 1)
Optical Power Relations
Many of the MAX3724/MAX3725 specifications relate to the input signal amplitude. When working with optical receivers, the input is sometimes expressed in terms of average optical power and extinction ratio. Figure 6 and Table 1 show relations that are helpful for converting optical power to input signal when designing with the MAX3724/MAX3725. (Refer to Application Note HFAN-3.0.0: Accurately Estimating Optical Receiver Sensitivity.)
Optical Linear Range
The MAX3724/MAX3725 have high gain, which limits the output when the input signal exceeds 50AP-P. The MAX3724/MAX3725 operate in a linear range (10% linearity) for inputs not exceeding: 50ARMS (re + 1) Linear Range = 10log 1000 dBm 2(re - 1)
Table 1. Optical Power Relations
PARAMETER Average power Extinction ratio Optical power of a 1 Optical power of a zero Signal amplitude SYMBOL PAVG re P1 P0 PIN PAVG = (P0 + P1) / 2 r e = P1 / P 0 P1 = 2PAVG(re) / (re + 1) P0 = 2PAVG / (re + 1) PIN = P1 - P0; PIN = 2PAVG(re - 1) / (re + 1) RELATION
Note: Assuming 50% average duty cycle and mark density.
8
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3.2Gbps SFP Transimpedance Amplifiers with RSSI
Layout Considerations
Noise performance and bandwidth are adversely affected by capacitance at the IN pad. Minimize capacitance on this pad and select a low-capacitance photodiode. Assembling the MAX3724/MAX3725 in die form using chip and wire technology provides the best possible performance. Figure 7 shows a suggested layout for a TO header for the MAX3724/MAX3725. The placement of the filter cap to minimize the ground loop of the photodiode is required to achieve the specified bandwidth. The OUT+ and OUT- bond wire lengths should also be minimized to meet the bandwidth specification. Special care should be taken to ensure that ESD at IN does not exceed 500V. Current generated by supply noise voltage is divided between CFILTER and CPD. The input noise current due to supply noise is (assuming the filter capacitor is much larger than the photodiode capacitance): INOISE = (VNOISE)(CPD) / (RFILTER)(CFILTER) If the amount of tolerable noise is known, the filter capacitor can be easily selected: CFILTER = (VNOISE)(CPD) / (RFILTER)(INOISE) For example, with maximum noise voltage = 100mVP-P, CPD = 0.85pF, RFILTER = 600, and INOISE selected to be 350nA: CFILTER = (100mV)(0.85pF) / (600)(350nA) = 405pF
MAX3724/MAX3725
Photodiode Filter
Supply voltage noise at the cathode of the photodiode produces a current I = CPD V/t, which reduces the receiver sensitivity (C PD is the photodiode capacitance.) The filter resistor of the MAX3724/MAX3725, combined with an external capacitor, can be used to reduce this noise (see the Typical Application Circuit).
Wire Bonding
For high-current density and reliable operation, the MAX3724/MAX3725 use gold metalization. Connections to the die should be made with gold wire only, using ballbonding techniques. Die thickness is typically 14 mils (0.4mm).
TOP VIEW OF TO-46 HEADER
PI
CASE GROUND
400pF TO 1000pF 400pF TO 1000pF VCC
OPTICAL POWER
PAVG
PHOTODIODE OUTOUT+
PO
TIME
MAX3724 MAX3725
Figure 6. Optical Power Relations
Figure 7. Suggested Layout for TO-46 Header
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3.2Gbps SFP Transimpedance Amplifiers with RSSI MAX3724/MAX3725
Chip Topography
GND
MAX3724 MAX3725
VCC
1
10
9
OUT+
N.C.
2 3 4 5 6 7 8
OUT0.03in (0.76mm)
VCC
IN
FILTER
GND 0.05in (1.26mm)
N.C.
Pad Coordinates
PAD 1 2 3 4 5 6 7 8 9 10 COORDINATES (m) X 1.4 0 0 0 0 494.2 865.2 1005.2 1005.2 490 COORDINATES (m) Y 495.6 336 224 112 0 -1.4 -1.4 -1.4 495.6 495.6
Chip Information
TRANSISTOR COUNT: 301 PROCESS: SiGe Bipolar SUBSTRATE: ISOLATED DIE THICKNESS: 0.014in 0.001in
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
10 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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